Modern high power gyrotrons produce power in high-order TE modes (TEmn modes with m,n>>1). These modes cannot be efficiently transported as RF (radio frequency) power in a low loss transmission system. In addition, it is advantageous to separate the RF transmission power from that of the spent electron beam within the gyrotron. Both of these considerations are typically addressed using an internal mode converter and a step-cut launcher, which is commonly referred to as a quasi-optical (QO) launcher. The mode converter has small deformations in the input waveguide surface to transform the high-order cavity mode into a set of modes whose combined fields have a Gaussian-like profile. The Gaussian-like beam can then be efficiently launched, focused, and guided by mirrors inside the vacuum envelope of the gyrotron. In this way, the propagating RF power is converted to a mode more suitable for low loss transmission, and the propagating RF beam is separated from the electron beam which carried it. This allows implementation of a depressed collector with large surfaces for thermal dissipation without affecting the quality of the RF beam.
This method has been the primary technique for RF-electron beam separation in high power gyrotrons since the early 1990s. The development of this technique was one of the key technologies enabling the development of mega-watt (MW) level gyrotrons. One drawback of this approach is the internal mirrors must be adjustable for optimum performance to prevent device overheating from internal losses at the high power levels. Additionally, since these large mirrors are external to the gyrotron cavity, the RF power must be coupled out of the gyrotron through a large aperture, which is typically fabricated from expensive materials such as diamond which have the desired low RF power loss and high thermal conductivity required. There are several deficiencies in this technique including internal diffraction losses, electron beam potential depression, and mirror alignment issues.
It is desired to provide a mode converting device which converts high order WG modes travelling helically in a cylindrical input waveguide into HE11 mode for coupling into a corrugated output waveguide inside the gyrotron, thereby greatly reducing the deficiencies of the prior art approaches. In addition, substantial cost savings can be realized by eliminating the need for the two to three adjustable mirrors in the gyrotron and the external mirror optical unit used to couple the output Gaussian beam to the corrugated output waveguide transmission line. A final cost savings would be realized by the large reduction in the required diameter of the diamond material in the output window.